Question

所以我在C ++项目中添加了lambdas的使用，现在编译器提供了这样的输出：

9>C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xxcallobj(13): warning C4800: 'BOOL' : forcing value to bool 'true' or 'false' (performance warning)
3>          C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xxfunction(386) : see reference to class template instantiation 'std::tr1::_Impl_no_alloc0<_Callable,_Rx>' being compiled
3>          with
3>          [
3>              _Callable=_MyWrapper,
3>              _Rx=bool
3>          ]
3>          C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xxfunction(369) : see reference to function template instantiation 'void std::tr1::_Function_impl0<_Ret>::_Reset0o<_Myimpl,_Fty,std::allocator<_Ty>>(_Fty,_Alloc)' being compiled
3>          with
3>          [
3>              _Ret=bool,
3>              _Fty=`anonymous-namespace'::<lambda3>,
3>              _Ty=std::tr1::_Function_impl0<bool>,
3>              _Alloc=std::allocator<std::tr1::_Function_impl0<bool>>
3>          ]
3>          C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\functional(113) : see reference to function template instantiation 'void std::tr1::_Function_impl0<_Ret>::_Reset<_Fx>(_Fty)' being compiled
3>          with
3>          [
3>              _Ret=bool,
3>              _Fx=`anonymous-namespace'::<lambda3>,
3>              _Fty=`anonymous-namespace'::<lambda3>
3>          ]
3>          ..\..\Common\IOFile.cpp(1162) : see reference to function template instantiation 'std::tr1::function<_Fty>::function<`anonymous-namespace'::<lambda3>>(_Fx)' being compiled
3>          with
3>          [
3>              _Fty=bool (void),
3>              _Fx=`anonymous-namespace'::<lambda3>
3>          ]

我的lambda的代码是：

在.h：

typedef std::function<bool ()> RepeatingFunction;
static bool RepeatFileOperation(RepeatingFunction callback);

static bool Copy(CString file, CString copyFileName, bool failIfExists = true);

在.cpp：

中

bool IOFile::RepeatFileOperation(RepeatingFunction callback)
{
    const int times_to_retry = 10;
    bool succeed = false;

    // Retry a few times if it doesn't work
    int retries = 0;
    do
    {
        // Perform the caller's action on the file
        succeed = callback();
    }
    while (!succeed && retries++ < times_to_retry);

    return succeed;
}

bool IOFile::Copy(CString file, CString copyFileName, bool failIfExists)
{
    return RepeatFileOperation([&] {
        return CopyFile(file, copyFileName, static_cast<BOOL>(failIfExists));
    });
}

该程序仍然编译得很好。我用谷歌搜索错误，可以找到人们得到类似的消息，但在他们的情况下，该程序不会构建。在所有情况下，它们的构建错误似乎都是关于前向声明，但正如我所说，我的构建很好，我在头文件中包含<functional>，所以它应该能够找到它。

这些消息是我应该担心的还是他们只是预期的行为？

Answer 1

CopyFile没有返回bool，但您隐式将其结果投射到bool。只需添加显式广告return static_cast<bool>(CopyFile(...

即可

在C ++中使用lambdas时出现奇怪的编译器消息

1 个答案: